If you want to tow heavy you should get you individual axle weights from the truck. Full gas, family, stuff...etc..
Then take those weight and subtract from dor tag ratings..
GAWR = 3950 lbs (both front & rear axles)
This will be more accurate..
But other things to consider..
1. What are the tire ratings at max air pressure? This may be a limiting factor.
2. Yes you need a WD hitch but first need to weigh trucks individual axles to see what your dealing with..
3. Find the weakest link to the truck.. This would most likely be the transmission. I would suggest installing the biggest trans cooler you could fit behind the grill.
4. Tell us the exact model truck you have.. 4 WD? Ext cab? Long bed? etc...